Charged particle emission such as electron emission from solid surfaces may be used to advantage in an imaging mode. In particular if an energy filtered image is produced the chemical morphology of a surface or the information about the excitation processes can be determined.
An engery filtered image is constructed from electrons with energies centered about E.sub.o and a band-pass of +.delta.E. Techniques for producing energy filtered image fall into the following three catagories.
(1) Those in which the excited area on the object is much smaller than the collection area of the analyser aperture (the microprobe method). Examples of this group are Auger electron spectroscopy and backscattering in the scanning electron microscope.
(2) Those in which the area of the analyser aperture is much smaller than the excited area on the object (the selected area method). Examples are field-emission spectroscopy and the scanning photoelectron spectroscopies.
(3) Those in which the area of excitation (or its image) is much the same as the area of the analyser aperture (the whole-image method), for example the energy-selecting electron microscope and the magnetically-collimated emission spectromicroscope.
This disclosure is concerned with an imaging band-pass analyser for whole-image energy-filtering. Whole image techniques provide rapid signal collection over the total image area and thus can be particularly time efficient.
One problem in designing fixed band-pass energy analyser which operate in a magnetic field, is that there are no elementary transmission low-pass energy filters which will preserve an image. Consequently a retarding-field electron mirror is used which can in principle maintain the image geometry. To produce a usable filter however, the reflected filtered electron stream must be separated from the incoming stream. This can be achieved by using crossed electrostatic and magnetic fields in the region preceding the filter. In a cross-field region, electrons acquire a drift velocity, perpendicular to both field directions, irrespective of the initial direction of motion of the electrons. Consequently, electrons can be drifted into the electron mirror and out again along different paths.
Transmission high-pass energy filters based on electrostatic saddle fields are well known and capable of very high energy discrimination when applied to narrow beams. For uniform retardation across a wide stream, in the absence of a magnetic field, planar grids are needed with consequent obstruction of the image and sensitivity to the spatially varying micropotentials. When a magnetic field is coaxial with the electrostatic saddle field the effects of off-axis transverse electrostatic field components are largely suppressed, as will be discussed later. Thus it becomes possible, in principle, to dispense with grids.
A band-pass energy filter can therefore be constructed be directing the image electrons into an electron mirror then, using crossed field deflection, to a high-pass filter. Using a single electron mirror however results in a reversal of the beam direction in the analyser. The use of a second electron mirror, either before of after the high-pass filter, restores the direction of the initial beam. An advantage of using such a folded electron beam path is that the length of the analyser is reduced and it requires a smaller region of uniform magnetic field in which to operate.
As mentioned the low-pass electron mirror necessitates the use of crossed electrostatic and magnetic fields which serve to transport the image electrons from one stage of the analyser to the next. Unfortunately, in uniform crossed fields, this arrangement suffers from a severe defect. The drift imposed by the crossed fields is a function of the time that the electrons spend in the fields, consquently different energy electrons having different forward velocities are thus deflected by different amounts. This combined field and energy-dependent drift has two consequences for the image.
When the image is passed through the cross-field region the electrostatic field gradient across the entrance aperture, due to the electrostatic deflection field, causes the image electrons to be accelerated differently according to their position in the aperture. Those nearest the positive deflection plate are accelerated more than those near the negative one and hence gain forward velocity. Because of their greater forward velocity they spend less time in the deflection field and are displaced less than the slower electrons. This causes a non-linear shear of the image. The larger the voltage drop across the aperture the greater the magnitude of this effect.
Another consequence arising from the use of crossed fields with images is the energy-dependent displacement of the final image. Image electrons of different energy, even if they enter the crossed-field region at the same initial position, will be displaced by different amounts. This dispersion variation causes the different energy components of adjacent areas of the image to be superimposed into a composite image. This results in a smeared image, unless the band-pass width is very small.
In summary then the method of image transport through crossed fields introduces unwanted effects of non-uniform image shear and energy dependent dispersion, and it is an object of the present invention to correct for these deficiencies.
According to one aspect of the invention there is provided a method of providing an energy filtered charged particle image comprising, applying a crossed electrostatic and magnetic field to a beam of imaging charged particles to cause deflection thereof from an initial path of travel, reflecting the deflected beam in said field to effect energy filtering thereof, providing an output beam of energy filtered imaging charged particles from said reflecting travelling in a path displaced from said initial path, providing a non-uniform electrostatic field of the said crossed field, and configuring said non-uniform electrostatic field whereby to control motion of the charged particles in the crossed field and correct for distortion of the image in said output beam.
Employing the use of a non-uniform electrostatic field in the crossed field region as defined above, results in the setting up of electrostatic field components acting transverse to the direction of the applied magnetic field of the crossed field and whose magnitude varies with transverse distance. This variation accordingly allows the drift velocity in the direction orthogonal to those fields to be controlled at each transverse distance.
Accordingly this allows the time that the imaging particles spend in the crossed field to be controlled and the local transverse motion to be simultaneously controlled. By careful choice of the form of the non-uniform electrostatic field, the displacement function may be arranged so that image shear is removed over much of the image.
Non-uniform electrostatic fields to provide the effect required may be produced in a number of ways preferably, for example, by shaped plates to define the equipotentials required, selectively perforated or segmented plates or separately potentially biased wires.
Such preferred means will be described in more detail in the description of a preferred embodiment of the invention which follows later in this disclosure.
To further enhance the distortion correction, a second crossed field may preferably be employed equivalent to the first field, but with the difference that the sense of the second crossed field direction is reversed with respect to the first field such that the deflection occurring in the first field is wholly restorable by deflection in the second field.
The second crossed field therefore serves to coalesce the various images back into registration and correct any remaining image shear in the imaging particles from the first field.
As is applicable to a single pass through the first field only, eventual success in shear correction using two passes, will be dependent on the design of the non-uniform electrostatic fields employed.
The fields required will depend on the coordinate system in which the deflection is defined. In a rectilinear coordinate system in which the image must be transported laterally without rotation, fields which are in variant in the direction of motion will be required. An alternative arrangement would use a cylindrical coordinate system in which image motion occurs around a circle with concomitant rotation about its own centre.
According to another aspect of the invention there is provided a device for providing an energy filtered charged particle image comprising an entrance aperture for an input beam of imaging charged particles, means for forming a crossed electrostatic and magnetic field to deflect said input beam from an initial path of travel, means for reflecting the deflected beam to effect energy filtering thereof and provide an output beam of energy filtered imaging charged particles travelling in a path displaced from said initial path, and means for providing non-uniformity to said electrostatic field of the crossed field whereby to control motion of the charged particles in the crossed field and correct for distortion of the image in said output beam.
Other features and advantages of the present invention will become apparent as this disclosure proceeds to set forth more detailed ways and means of performing the invention and putting some into practical effect.